Re: Partitioning and postgres_fdw optimisations for multi-tenancy

On Thu, Jul 16, 2020 at 8:56 PM Andrey Lepikhov
<a(dot)lepikhov(at)postgrespro(dot)ru> wrote:
> On 7/16/20 9:55 AM, Etsuro Fujita wrote:

> >>>> On Tue, Jul 14, 2020 at 12:48 AM Alexey Kondratov
> >>>> <a(dot)kondratov(at)postgrespro(dot)ru> wrote:
> >>>>> Some real-life test queries show, that all single-node queries aren't
> >>>>> pushed-down to the required node. For example:
> >>>>>
> >>>>> SELECT
> >>>>> *
> >>>>> FROM
> >>>>> documents
> >>>>> INNER JOIN users ON documents.user_id = users.id
> >>>>> WHERE
> >>>>> documents.company_id = 5
> >>>>> AND users.company_id = 5;

> > PWJ cannot be applied
> > to the join due to the limitation of the PWJ matching logic. See the
> > discussion started in [1]. I think the patch in [2] would address
> > this issue as well, though the patch is under review.

> I think, discussion [1] is little relevant to the current task. Here we
> join not on partition attribute and PWJ can't be used at all.

The main point of the discussion is to determine whether PWJ can be
used for a join between partitioned tables, based on
EquivalenceClasses, not just join clauses created by
build_joinrel_restrictlist(). For the above join, for example, the
patch in [2] would derive a join clause "documents.company_id =
users.company_id" from an EquivalenceClass that recorded the knowledge
"documents.company_id = 5" and "users.company_id = 5", and then the
planner would consider from it that PWJ can be used for the join.

Best regards,
Etsuro Fujita